1. Field of the Invention
This invention relates to the interconversion of electric power between the alternating current (AC) and direct current (DC), forms. More particularly, this invention relates to electrical power conversion through the use of static devices to avoid the use of rotating machinery while minimizing distortion of the input and output power.
2. Description of the Background Art
The conversion of electrical power is typically accomplished by discrete electric components such as rectifiers, thyristors, inductors and capacitors or by rotating machinery.
More particularly, AC to DC conversion is predominately accomplished by rectifying the alternating current to produce a pulsating direct current. Unfortunately, pulsating direct current is satisfactory only for limited applications such as battery charging. When smoother direct current is needed, such as for most electronic applications, the pulsating DC is filtered by means of capacitors or inductors.
Since rectifiers intermittently draw current from the AC supply, harmonic current distortion and low power factor are unavoidable. Furthermore, when capacitive filters are used, current distortion further increases. Thus, additional filtering is usually required to prevent harmonic current distortion, particularly for compliance with U.S. Navy specification (MIL-E-16400) which prohibits harmonic current distortion in shipboard equipment.
The input and output waveform distortions discussed above are typically avoided through the use of a motor-generator set or a synchronous converter, both of which can convert AC to DC. Unfortunately, the expense, size, weight, noise and high maintenance of such machines precludes their wide use.
In regard to conversion of electrical power from DC to AC, electronic inverters are typically used. The inverters produce a square wave output waveform which inherently contains harmonics that result in increased losses and generally poorer operation than if the output was a pure sine wave. Improved inverters have been developed which attempt to approximate a sine wave through the use of six pairs of controlled rectifiers thereby producing an alternating square wave output crudely approximating a sine wave and whose frequency is determined by the firing of the controlled rectifiers. Unfortunately, these six step inverters also draw DC current intermittently rather than continuously.
In some applications, it is desirable to convert power from AC to DC during one time frame and then vice versa at another time. The converter used to convert AC to DC at one time and DC to AC at another is known as a regenerative or bilateral converter. Regenerative converters are useful in electric railways where it is desirable to recover the energy absorbed in decelerating the train and feed it back into the power system. However, electronic regenerative converters produce distortion of the input and output power for basically the same reasons discussed above. Furthermore, regenerative converters may involve as much as two rectifiers and two inverters with a DC link.
Therefore, it is an object of this invention to provide an apparatus and method which overcomes the aforementioned inadequacies of the prior art devices and methods and provides an improvement which is a significant contribution to the advancement of the electric power converter art.
Another object of this invention is to provide an electric power converter for converting alternating current to direct current without the use of rotating machinery such as that found in a motor-generator set or a synchronous converter.
Another object of this invention is to provide an electric power converter for converting AC to DC which continuously draws current from the AC supply (as opposed to intermittently) to minimize harmonic current distortion and low power factor.
Another object of this invention is to provide an electric power converter converting AC to DC having a pulsating output voltage waveform which can be capacitatively or inductively filtered while still minimizing harmonic current distortion and low power factor.
Another object of this invention is to provide an electrical power converter for converting AC to DC to produce an output voltage waveform with less ripple than that achieved by prior art converters.
Another object of this invention is to provide an electric power converter for converting DC to AC as a function of input frequency.
Another object of this invention is to provide an electric power converter to function as a DC to DC converter while minimizing distortion.
Another object of this invention is to provide an electric power converter functioning as a regenerative, bilateral converter.
Another object of this invention is to provide an electric power converter for converting any number of phases greater than one to any other number of phases.
Another object of this invention is to provide an electric power converter functioning as a phase shifter.
Another object of this invention is to provide an electric power converter functioning as a voltage adjuster.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.